Introduction to Ground Water Hydrology

Dr. Mohsin Siddique

Assistant Professor

Dept. of Civil & Env. Engg

1

Hydrological Cycle

2Earth’s Water Cycle or Hydrologic Cycle

Ground Water Hydrology

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� Groundwater is water that is stored in a porous media (soil/sand/gravel) under the soil surface.

� Ground water hydrology may be defined as the science of occurrence, distribution and movement of water below the surface of earth.

Ground Water Hydrology

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Shallow groundwater:

Quick recharge (weeks, months, years)

More prone to outside contamination (organic pollution, effluents)

Deep groundwater:

Slow recharge (decades, centuries, fossil)

Sometimes natural contamination (salts, fluor for instance)

Ground Water Hydrology

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� Saturated / Unsaturated zone

� The porous media can be fully filled with water, called the saturated zone

� The porous media can be partially filled with water, and partially with air,

called the unsaturated zone

� Groundwater management usually only considers the saturated zone

(unfortunately)

Aquifer

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� Water holding media: Aquifer (Unconfined, Confined or Semi Confined)

� Confining layer: Aquitard and aquiclude

Aquifer

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� Aquifer is a geologic unit that stores and transmits water.

Aquifer is a underground “reservoir”.

An aquifer can be confined, semi-confined or unconfined

� Confined: (also called artesian) Groundwater is between two restricting layers

� Unconfined: Top of the groundwater is not confined by a restrictive layer

� Semi-confined: Groundwater has semi-restrictive layer

� Aquitard and aquiclude is the name for a confining layer. A confining layer

restricts water flow (usually clay or bedrock)

� Aquitard (partly permeable, sandy clays)

� Aquiclude (contains water but does not transmit it, clays)

Water Table

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� Top of an unconfined aquifer is the water table

� Top of the pressure level in a confined aquifer is the phreatic level

or piezometric level or potentiometric surface

Water Table and Wells

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� Top of an unconfined

aquifer is the water

table

� Top of the pressure

level in a confined

aquifer is the phreatic

level or piezometric

level

Water Table

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� Piezometric level is HIGHER than the actual water level in the confined

aquifer !

� The pressure at the top of the water table equals the atmospheric pressure

� In groundwater hydrology, the pressure at the top of the water table is

defined as 0

� Thus: To lift water ABOVE the water table, energy is needed !!

Wells

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Groundwater well

� To measure the level of the water table

Piezometer

� To measure the level of the piezometric or phreatic level

Pumping well

� To extract water from the groundwater to the surface (a pump needs

energy to lift the water)

Physical Properties That Affect Groundwater

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Physical Properties That Affect

Groundwater

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� Porosity:

� The ratio of voids in a rock or sediment to the total volume of material is referred to as porosity and is a measure of the amount of groundwater that may be stored in the material.

� Permeability:

� It measures the transmission property of the media and the interconnection of the pores.

� It is related to hydraulic conductivity and transmissivity.

Darcy’s Law and Hydraulic Conductivity

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� Henry Darcy, a French hydraulic engineer, observed that the rate of laminar flow of a fluid (of constant density and temperature) between two points in a porous medium is proportional to the hydraulic gradient,i, (=dh/dl) between the two points (Darcy 1856).

� The equation describing the rate of flow through a porous medium is known as Darcy’s Law and is given as:

Q = K i A

i=dh/dl� Q = volumetric flow rate [L3T-1]

� K = hydraulic conductivity [LT-1]

� A = cross-sectional area of flow [L2]

� h = hydraulic head [L]

� l = distance between two points [L]

� Darcy law is valid only for laminar flow (Re= <= 1)ν

10VD

Hydraulic Conductivity & Transmissivity

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� Hydraulic Conductivity is the flow rate through a porous medium for unit cross sectional area under a unit hydraulic gradient.

k=Q/iA� Hydraulic conductivity is usually expressed in units of length/time: feet/day,

meters/day, or gallons/day/square-foot.

� Transmissivity, T ,is a measure of the amount of water that can be transmitted horizontally through a unit width by the fully saturated thickness of an aquifer under a hydraulic gradient equal to 1.

� It is equal to the hydraulic conductivity multiplied by the saturated thickness of the aquifer and is given by:

Ground Water Management

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� For sustainability, we must have a balance between Ground Water Withdrawal and Ground Water Recharge.

� If groundwater is pumped faster

than the recharge, water levels

drop

� Porous media lose water, pores are

filled with air

� Porous media could consolidate,

resulting in land subsidence, sink

holes, loss of water storage

capacity

land level in 1925

land level in 1955

land level in 1977

Artificial Aquifer Recharge

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� Artificial aquifer recharge (AR) is the enhancement of natural ground water supplies using man-made conveyances such as infiltration basins or injection wells.

Aquifer Storage and Recovery

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� Aquifer storage and recovery (ASR) is a specific type of AR practiced with the purpose of both augmenting ground water resources and recovering the water in the future for various uses.

Aquifer Storage and Recovery-UAE

19 LIWA ASR Project

Aquifer Storage and Recovery-UAE

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LIWA ASR Project

Aquifer Storage and Recovery-UAE

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LIWA ASR Project Inset Map

� Source:http://www.environmentalatlas.ae/cartography/resourceOfLife/waterInfrastructureDesalinationSupplyDistributionAndASRProjects

Thank you

� Questions….

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